Some showerheads work much better than others. By law, you cannot buy a head that flow more. Now, on some of them, you can remove the restrictor (often just a disk on the inlet). Think about buying a new showerhead. Make sure that the inlet screen of the head is not clogged, first, though. At only 1-1/2 years, it shouldn't be all limed up, but could easily have junk from contruction clogging the screen, or maybe sand if you are on a well.

It has nothing to do with the cartridge. However, some manufacturers do put a restrictor in the body to reduce flow to the shower. It is not to compy with the law, which does not limit the valve per se. It is to ensure that in a tub/shower combo all the water goes to the tub spout when desired. The restrictor ensures that water does not migrate up to the shower head in the 'tub' mode. The restrictor is not accessible without opening up the wall.

In a shower only install, plumbers will sometimes install the valve body upside down to eliminate this problem.

It seems to me (unscientific analysis here) that the flow in 2.5gpm head does drop off considerably if the house pressure is low. Check to see that you have at least 60PSI static, and that it doesn't drop below fifty when using the shower.

You have good pressure, but 2.5gpm flow with no shower head is low. By any chance does the shower valve have screwdriver stop valves on the water inlets? ( Remove the round escutcheon flanvge to see). If so, ensure they are completely open.

This is messy, but you could remove the shower valve cartridge and then have some one momentarily turn the water back on. This should be a gusher coming straight out at you!......at least it would confirm the location of the problem.

All the cartridges are the same and if you remove the one from your old house you may damage it and then have to buy another one. The water flow inside the valve passes through one, or two, "ports" into the lower chamber which would normally be connected to a tub diverter spout. There is also a "port" which goes back up through the valve to carry water to the shower head when the diverter is pulled, or it is a shower only installation. This port is usuallly smaller than the one(s) going down to the spout and that is probably where your restriction is. It cannot be changed or increased, which is why I install the valve upside down, (as long as the valve is symetrical so it can be done), and gives the shower head the full flow from the control mechanism.

My house just has 1/2" copper throughout so low flow is always a concern. I just put in the new shower and wish I'd know about turning the valve upside and using the tub side.
But wondered if it was a huge hassle to do it now.
I can't think of a way to do it without sweating the elbows off.
See the attached photo.
Then I wondered how you deal with handle/control. Can you twist ot 180 degress or do have to push down instead of lift up. If you rotated it wouldn't the hot be on the wrong side then? I guess you'd plan ahead you'd run the pipes to different sides.
Oh well probably to much hassle to flip it over now.

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Before doing it you have to be sure that the valve and trim is symetrical so it can be inverted without affecting the appearance. Most single handle valves have internal parts that can be rotated to interchange the hot and cold sides, but if not the hot and cold pipes are just rerouted to the opposite sides from normal. All of this is determined before installing the valve. At this time inverting it would be a fairly labor intensive job since it would have to be removed and the reconnected. The easier way would be to remove the bottom plug and run a pipe from there and connect it to the shower riser with a tee so you get flow from both the top and bottom of the valve. This would give you the maximum flow possible.

I like that idea of teeing in a connection from the tub outlet of the valve.
That does sound simple and is likely to give the max pressure in any case. Does that mean that the shower will get flow from both the shower and tub outlet at the same time?
For future reference, just curious if any of the major mfr. make a shower only valve that doesn't have this reduced pressure issue?

When you look at their specs, it becomes more obvious. Yes, some shower valves are designed to give good flow to numerous sprays at once. Most that do that require a 3/4" supply, though. A more typical 1/2" supply doesn't usually provide a great multiple head result (adequate maybe). My unprofessional opinion.

Most do, because they only have to furnish the 2.6 gpm flow so there is no reason to give a greater flow. And since the flow has to pass back up through the valve to the shower, there is only a limited amount of space for the shower port. As an adjunct, I have not installed a Moen valve for years, but it is one that cannot be inverted because the plate and valve are not symetrical, so teeing the discharge, or using the lower port and piping it back up to the shower head is the only way to provide maximum flow.